Because when they knew and recognized Him as God, they did not honor and glorify Him as God or give Him thanks. But instead they became futile and godless in their thinking [with vain imaginings, foolish reasoning, and stupid speculations] and their senseless minds were darkened. Romans 1:21 (Amplified Bible Classic Edition, AMPC) emphasis mine

This is a hard-hitting verse. If you read it carefully, it is speaking of the fact that God reveals Himself to all people- they knew and recognized Him as God. However, they did not give honor to God by thanking Him. They took their lives and the good things they received for granted, even thinking they somehow did it themselves. Failing to give God thanks led to godless thinking. And godless thinking led to darkened minds. In other words, they did not acknowledge God as the source of the good they received, and as a result, their thinking became more and more like the enemy’s thinking- darkened. Let’s look at it in context, including the verse before and after.

For ever since the creation of the world His invisible nature and attributes, that is, His eternal power and divinity, have been made intelligible and clearly discernible in and through the things that have been made (His handiworks). So [men] are without excuse [altogether without any defense or justification], Because when they knew and recognized Him as God, they did not honor and glorify Him as God or give Him thanks. But instead they became futile and godless in their thinking [with vain imaginings, foolish reasoning, and stupid speculations] and their senseless minds were darkened. Claiming to be wise, they became fools [professing to be smart, they made simpletons of themselves]. Romans 1: 20-22 (Amplified Bible Classic Edition, AMPC)

Failing to give God thanks leads people down a road that leads to self-deception. Because they did not acknowledge the Lord or give Him thanks, they became fools- thinking they were so smart but in reality being stupid. That is the ultimate deception- when we believe we are the source of all our blessings and more intelligent than the God who made all things.

Regular giving of thanks to God will help keep us from growing cold in our love for God and falling away from the faith. When we thank Him in all situations, our focus is kept on Him. It keeps us humble and centers our hearts on the Lord. When people constantly thank God, it is difficult for them to be led away from God. Thanksgiving is a form of protection for us.

Thank [God] in everything [no matter what the circumstances may be, be thankful and give thanks], for this is the will of God for you [who are] in Christ Jesus [the Revealer and Mediator of that will]. I Thessalonians 5:18 (Amplified Bible Classic Edition, AMPC)